(1) Prior Art
In the process of the preparation of polyisocyanates (e.g. urethane prepolymers) which are used in conjunction with water in the production of polyurethane foams, polyoxyethylene polyols are generally end capped with monomeric isocyanates.
One of the monomeric isocyanates extensively used industrially in said end capping reaction is tolylene diisocyanate hereinafter referred to as TDI. TDI, as isocyanates in general, has been known for its physiologically detrimental effects and although generally only 2-3% by weight of free TDI is present in the polyoxyethylene polyol-TDI reaction product, which is used in foam manufacturing industry, it is sufficient to constitute a serious hazard to production line workers who are exposed to large volumes of the volatile isocyanate for prolonged periods of time.
Recent OSHA regulations have reduced the amount of free TDI permitted in the air from 0.02 p.p.m. to 0.005 ppm.
The prior art relating to means of removing residual free TDI from polyoxyethylene polyol-TDI reaction product is scarce which is attributable to the fact that only recently has a consciousness developed with regards to the deleterious effects of the presence of relatively minute quantities of unreacted TDI in said reaction product.
Nevertheless a number of expedients have been proposed in the past for rendering the isocyanates, and particularly volatile diisocyanates, utilizable. In particular, it has been proposed to react volatile di-functional isocyanates (having the general formula R-[NCO].sub.2) with polyfunctional alcohols (having the general formula X(OH.sub.n)), in a ratio --NCOeq/--OHeq of about 2:1.
Theoretically, that reaction should result in the production of a high molecular weight (and consequently essentially non-volatile) polyisocyanate containing as many --NCO isocyanate groups as there are hydroxyl groups contained in the polyalcohol, i.e. in the production of products of the type: ##STR1##
However, in practice, that result is not obtained. Using the aforesaid ratio, there still remain, in the final product, hazardous unreacted amounts of the starting volatile di-functional isocyanate and there are present compounds having a higher molecular weight than the theoretical molecular weight and which derive from the reaction of all the isocyanate groups of the same molecule of the same starting volatile difunctional isocyanate.
It is possible, by using --NCOeq/--OHeq ratios higher than 2:1, to reduce the formation of high molecular weight condensates. However, when such ratios are used, the amount of unreacted volatile diisocyanate contained in the reaction product is increased.
On the other hand, when --NCOeq/--OHeq ratios lower than 2:1 are used, the amount of unreacted volatile diisocyanate in the reaction product is reduced, but the content of high molecular weight condensates is increased.
According to the prior art, it is, in general, preferred to use --NCOeq/--OHeq ratios around 2:1 and to then remove the unreacted volatile isocyanate by chemical or physical means.
According to one prior art method, the unreacted volatile isocyanate is removed from the crude (total) reaction product by rapid distillation under vacuum. That procedure involves a number of drawbacks since, in order to insure a rapid distillation, it is necessary to operate at temperatures that give collateral reactions which (by reaction between isocyanic and urethane groups) result in the formation of allophanic acid esters and polymers having an isocyanate structure. The result is that the finished product obtained by that process is (as compared to the crude reaction product) strongly yellowed, more viscous, and has a higher molecular weight and a lower content of isocyanic group.
According to another prior art method, the crude (total) product of the reaction between the isocyanate and polyfunctional alcohol is treated with an aliphatic or cycloaliphatic hydrocarbon which is a selective solvent for the unreacted volatile diisocyanate. The treatment with the selective aliphatic or cycloaliphatic hydrocarbon must be carried out at a starting temperature of at least 80.degree. C. and at a final temperature of about 130.degree. C. to prevent precipitation of the high molecular weight polyisocyanate during extraction of the unreacted volatile isocyanate. Owing to the temperatures used, that procedure involves the same drawbacks as those mentioned previously.
According to another prior art method, the unreacted volatile diisocyanate is extracted from a solution of the crude (total) reaction product, in a solvent such as, for example, acetic acid esters, alkyl carbonates, ketones, chlorinated hydrocarbons which are, in general, the same as the solvents used as the liquid reaction medium, with mixtures of those solvents of the crude reaction product and aliphatic or cycloaliphatic hydrocarbons. The high molecular weight polyisocyanate is only slightly soluble in the aliphatic or cycloaliphatic hydrocarbons which show an affinity for both the unreacted volatile isocyanate and the solvents for the crude reaction product, so that, using this method it is possible to carry out the extraction of the unreacted volatile isocyanate at temperatures which avoid the collateral reactions mentioned hereinabove.
However, this last mentioned method is not free from technological difficulties. In practice, the mixture of solvents used for the extraction of the unreacted isocyanate from the crude reaction product must be selected in dependence on the nature of the polyisocyanate and used in a particular ratio of the solvent for the particular crude reaction product to the aliphatic or cycloaliphatic hydrocarbon which ratio must be maintained constant throughout the extraction. For example, if an excess of the solvent for the crude reaction product is used, there is a significant loss of high molecular weight polyisocyanate during the extraction, while use of an excess of the aliphatic or cycloaliphatic hydrocarbon results in the precipitation of the high molecular weight polyisocyanate in the form of crystals which obstruct and block the extraction apparatus.
Most recently a process for the obtention of a high molecular weight polyisocyanate has been disclosed in U.S. Pat. No. 3,883,577 where the reaction between the volatile diisocyanate and an active hydrogen containing compound is carried out in a solvent medium which has a pg,7 strong affinity for the reaction product (high molecular weight polyisocyanate) but is only partly miscible with the hydrocarbon used as solvent for the extraction of the unreacted volatile diisocyanate and facilitates separation of the diisocyanate by being easily distillable without causing collateral reactions.
It may thus seem that the prior art has dealt with the problem of removal of unreacted TDI from polyisocyanates by primarily concentrating on either stoichiometric adjustments or solvent extraction, both methods having as main drawbacks the inherent undesirable effect on the reaction products' molecular weight (and on crosslink density).